This invention relates to a light-exposure apparatus with a thin plate deforming mechanism suitable either for flattening a thin plate such as a silicon wafer, bubble wafer, ceramics substrate or printing board when the thin plate is desired to be exposed to light or for concavely bending the thin plate when an optical image is formed on that thin plate.
Conventionally, to manufacture a desired circuit in LSIs, for example, a resist film is coated on a silicon wafer, a desired pattern formed on a mask is transferred to the resist film, and such treatments as etching and ion implantation are repeated.
In the LSIs, it is required that the width of lines constituting a fine circuit pattern be 1 m or less in order to improve the degree of integration. To meet this requirement, the use of soft X-rays, for example, has been proposed for transfer of the mask pattern.
Of several proposals, an X-ray exposure apparatus using soft X-rays is disclosed in, for example, Japanese Patent Unexamined Publication No. 57-169242. When using soft X-rays for exposure, however, the soft X-rays generated from a generator greatly attenuate before reaching the resist film formed on the wafer, as well known in the art.
To cope with this problem, many improvements have been made by increasing the dose of soft X-rays generated from the X-ray generator, or making the mask from a material which is easy to transmit soft X-rays and minimizing the thickness of the mask.
Mechanical strength of the mask will be decreased in proportion to a decrease in its thickness and hence a thin mask of a large area is difficult to produce. Thus, in a proposed step and repeat transfer method, a thin mask is prepared for formation of one to several LSIs at the most, and a wafer is moved stepwise pattern by pattern so that patterns for individual LSIs may be sequentially transferred onto the wafer. The thin mask is however difficult to flatten.
On the other hand, the soft X-rays from the generator are radially divergent and propagate straight-forwardly. The source of soft X-ray generator has an area corresponding to the diameter or size of an electron beam which irradiates an anode electrode. Accordingly, the soft X-rays from the generator pass through the mask and reach different positions on the resist film formed on the wafer which cover the area of source of the generator, resulting in a blur or such a shift that the soft X-rays irradiate a position which is in slight misregistration with the mask pattern.
Incidentally, the mask is adversely affected by many factors such as errors in its manufacture, distortion due to temperature rise upon exposure, distortion due to its aging especially when the mask is used repetitiously, distortion due to gravitative weight of the mask when the mask is deformed by chucking during mounting it to the light-exposure apparatus, and distortion due to the difference between air pressures exerted on the top and bottom surfaces of the mask. The wafer is also affected adversely by many factors such as distortion caused in the course of its manufacture, distortion caused by chucking when mounting it to the light-exposure apparatus, and distortion caused during such processes as etching and ion implantation.
Accordingly, in order to transfer a circuit pattern of lines having a width of 1 .mu.m or less onto the wafer, a surface area of a portion of wafer to be exposed to light must be so deformed as to be placed in the best condition for receiving a pattern image projected from the mask.
An apparatus contrived to meet this requirement has been proposed in, for example, "Flatness Controlled Wafer Clamping Pedestal", IBM Technical Disclosure Bulletin, Vol. 15, No. 10, 1973. In this proposal, a pedestal is formed with holes for vacuum absorption and piezoelectric devices are disposed in association with the pedestal. When a wafer is placed on the pedestal and absorbed thereby by vacuum (vacuum absorbed), heights of the wafer top surface are detected at different points by means of a sensor and desired voltages based on results of detection are applied to the piezoelectric devices to push up the bottom surface of the wafer so that the top surface of the wafer may be flattened.
With this apparatus, however, a gap takes place between the wafer pushed up by the piezoelectric devices and the pedestal, with the result that the wafer clamping force by vacuum is weakened and the wafer tends to slip under the influence of an air current flowing through the gap. Further, unless the piezoelectric devices are arranged uniformly over the entire area of the wafer, the wafer can not be flattened or deformed into a desired shape. Thus, when the piezoelectric devices are arranged at 10 mm intervals over the entire area of a wafer of, for example, 4-inch diameter, the number of the piezoelectric devices amounts up to 105; for a wafer of 5-inch diameter, there needs 149 piezoelectric devices. The same number of drivers must be provided for the piezoelectric devices. Normally, the driver must apply voltages of 0 (zero) to 650 V to the piezoelectric device and such a driver is technically difficult to miniaturize. Therefore, equipping the light-exposure apparatus with a great number of drivers faces technical difficulties and besides, unnecessarily increases size and cost of the apparatus.
Another conventional apparatus is disclosed in Japanese Patent Unexamined Publication No. 59-106118 wherein piezoelectric devices adapted to deform a wafer are brought into contact with a wafer absorptive thin plate in evacuated atmosphere and expanded or contracted to partially deform the wafer absorbed to the wafer absorptive thin plate. This apparatus however calls for high density of arrangement of the piezo-electric devices for the sake of flattening the entire area of the wafer, resulting in complexity of its construction and high cost.